1. Field of the Invention
The present invention relates to an optical transponder in an optical transport system, and more particularly to an optical transponder with an add/drop operation function of optical channels, which combines operating functions of adding and dropping type optical transponders in accordance with mounting of a digital wrapper to simultaneously process two optical channels, thereby performing addition and detection of maintenance information and monitoring information and detecting errors generated during optical transfer of signals due to addition of error correction symbol signals, thereby recovering those signals
2. Description of the Related Art
Generally, optical transponders serve to transport signals received from an external transport network to an optical transport network, or to transmit signals received from the optical transport network to the external transport network. That is, such optical transponders serve as an access point between different transport networks while performing addition and detection of maintenance information and monitoring information of respective optical channel layers of optical signals received therein, in order to perform its maintenance and monitoring functions.
Also, such optical transponders are classified into those of an adding type which detects maintenance information and monitoring information contained in an optical signal transmitted from the external transport network, adds, to the optical signal, new maintenance and monitoring information suitable for operations of the external transport network, based on the detected result, and transmits the resultant optical signal, and those of a dropping type which detects maintenance information and monitoring information contained in an optical signal transmitted from a transport network, monitors the optical signal based on the detected information, adds, to the optical signal, new maintenance and monitoring information suitable for operations of the external network, based on the detected result, and transmits the resultant optical signal to the external network.
Thus, only one of the above mentioned two functions is performed by one optical transponder. Accordingly, it is necessary to use two optical transponders in order to perform both functions. This will be described in conjunction with FIG. 1.
FIG. 1 is a block diagram illustrating an optical transport system using typical optical transponders. Referring to FIG. 1, an optical signal transmitted from an optical transmitter 11 included in an external network (A) 10 is received by the adding type transponder part TPa of an optical transponder 211 included in an optical channel layer 21. The adding type transponder part TPa of the optical transponder 211 opto-electrically converts the received signal, detects maintenance information and monitoring information from the converted signal, performs desired maintenance and monitoring operations based on the detected information, adds, to the signal, maintenance information and monitoring information, electro-optically converts the resultant signal, and transmits the converted signal to an optical multiplexer 231 included in an optical multiplexing/demultiplexing layer 23. The optical channel layer 21 includes N optical transponders. In FIG. 1, only the first and N-th optical transponders are denoted by the reference numerals 211 and 212, respectively. Each optical transponder includes an adding type transponder part TPa and a drop type transponder part TPd. Thus, N optical signals respectively transmitted from the N optical transponders are multiplexed by the optical multiplexer 231 of the optical multiplexing/demultiplexing layer 23. The multiplexed signal outputted from the optical multiplexer 231 is amplified by an optical amplifier 251 included in an optical amplifier layer 25, and then transmitted via and optical line, that is, an optical fiber 26.
The optical signal transmitted via the optical fiber 26 is demultiplexed by an optical demultiplexer 241 included in an optical multiplexing/demultiplexing layer 24, so that it is split into signal components of different wavelengths respectively corresponding to different optical channels. Each of the split optical signals is opto-electrically converted by the drop type transponder part TPd of an associated optical transponder 221 included in an optical channel layer 22. The drop type transponder part TPd of the optical transponder 221 detects maintenance information and monitoring information from the optical signal, thereby performing maintenance and monitoring operations. Thereafter, the drop type transponder part TPd of the optical transponder 221 adds maintenance information and monitoring information to the optical signal, electro-optically converts the resultant optical signal, and transmits the converted optical signal to an external network (B) 30.
Meanwhile, signals transmitted from an optical transmitter 31 included in the external network (B) 30 are transmitted to an optical receiver 12 included in the external network (A) 10 after being processed through the same procedures as described above. As apparent from the above description, each transponder 211 or 221 of the optical channel layer 21 or 22 includes an adding type optical transponder part TPa, and a drop type optical transponder part TPd. That is, it is necessary to use two optical transponders respectively having optical adding and dropping functions in order to achieve both the optical adding and dropping operations.
FIG. 2 is a block diagram illustrating an optical transponder used in a conventional optical transport system. As shown in FIG. 2, the optical transponder includes an optical distributor 41, an optical receiver 42, a section overhead (SOH) processor 43, an optical transmitter 44, an optical detector 45, a monitoring signal detector 46, and a monitoring information processor 47.
An optical signal transmitted from an external network is received in the optical distributor 41 which branches the received optical signal. The branched optical signal is opto-electrically converted by the optical detector 45, so that it is converted into an electrical signal. From this electrical signal, signal components corresponding to maintenance and monitoring information added in the form of modulated low frequency components to the information signal are separated. Maintenance information and monitoring information are obtained from the separated signal, so that they are used for the maintenance and monitoring of the optical signal.
An overhead newly generated in the monitoring information processor 47 for maintenance and monitoring operations is applied to the SOH processor 43 arranged downstream from the monitoring information processor 47. The SOH processor 43 adds the signal modulated in the form of low frequency components by the monitoring signal processor 47 to the information signal received from the optical receiver 42.
As apparent from the above description, where the conventional optical transponder 40 is applied to an optical transport system, there is a problem in that it is necessary for the optical transponder to have two different transponder parts in order to obtain both optical adding and dropping functions. Furthermore, it is necessary to modulate the information signal in the form of low frequency components in order to allow the addition of maintenance information and monitoring information to the information signal. For this reason, the added information may act as noise in the information signal.
Various methods for detecting monitoring components from an optical signal have been proposed. For example, U.S. Pat. No. 5,383,046 discloses a supervisory and control signal transmitting system for use in an optically amplifying repeater system for amplifying light attenuated over a long distance between a transmitting station and a receiving station through a plurality of repeaters, and sub-carrier-converting supervisory information and control information in order to transmit those information in a state carried by an optical signal. This system includes a receiver, a supervisory information detector, a supervisory information processor, sub-carrier modulator, and an optical transmitter, in order to process supervisory signals through modulation of information signals. However, such a system has no function of correcting errors contained in information signals, currently transmitted, because signal detection and addition are performed only for the supervisory signals contained in optical signals being transmitted. Furthermore, it is difficult to achieve a desired maintenance for information signals. Where a transport network using the above mentioned system is constructed, it is necessary to construct two separate systems for maintenance and monitoring operations during the transmission of optical signals. In addition, there is a limitation on the transmission distance and transmission performance because the monitoring signals contained in optical information signals may act as noise.
Meanwhile, U.S. Pat. No. 5,938,309 discloses a wavelength division multiplexing (WDM) optical communication system with remodulators transmitting signals at different bit rates. This system performs multiplexing and demultiplexing operations for different optical signals to maintain the same transmission rate for those optical signals, thereby allowing combination of the signals. However, although this system achieves a change of transmission rate required to allow optical signals of different bit rates to be transmitted to an optical communication system at a transmission rate of 2.5 Gbps, it does not implement adding or dropping operations of maintenance and monitoring information contained in information signals. Furthermore, this system has no error correcting function. Since this system is unidirectionally used as an optical transmitter or an optical receiver, it has a problem in that it operates inefficiently in an optical transport network.